1. Field of the Invention
The present invention relates to a printed wiring substrate having electronic components embedded in a core substrate, and to a method for manufacturing the same.
2. Description of the Related Art
In response to recent requirements for high densification and high performance of a printed wiring substrate, a printed wiring substrate having electronic components embedded in a core substrate has been proposed.
For example, a printed wiring substrate 40 shown in FIG. 6 is configured such that dielectric layers 43 are formed on the corresponding front and back surfaces of a dielectric layer 41 via unillustrated wiring layers. An electronic component 45 is mounted on the first main surface of the printed wiring substrate 40. By means of a prepreg adhesive layer 47, an electronic component 44 is embedded in a through-hole 42 formed in the dielectric layer 41, the dielectric layer 41 being located at the center of the printed wiring substrate 40 in the thickness direction, and chip capacitors (electronic components) 46 are embedded in corresponding recesses 42a formed in the dielectric layer 41 and extending from the interior of the dielectric layer 41 to the front surface of the dielectric layer 41.
3. Problems Solved by the Invention
In the printed wiring substrate 40, the chip capacitors 46 are embedded in the corresponding recesses 42a by the thin prepreg adhesive layer 47. Thus, a crack is likely to be generated in the prepreg adhesive layer 47 in the vicinity of an electrode of the chip capacitor 46, which extends through the prepreg adhesive layer 47. Such a crack impairs dielectric capability and hermetic capability in a region peripheral to the crack, and the characteristics of the chip capacitor 46 may become unstable.
It is therefore an object of the present invention to solve the above-mentioned problems of a conventional printed wiring substrate and to provide a printed wiring substrate having an electronic component embedded in a core substrate in a condition unlikely to generate cracks, as well as to provide a method for manufacturing the same.
To achieve the above object, the present inventors investigated the addition of an inorganic filler to a resin which is used to embed an electronic component, as well the particle size of the organic filler relative to an electrode of the electronic component, to thereby achieve the present invention.
Specifically, in a first embodiment, the printed wiring substrate of the present invention comprises a core substrate having a front surface and a back surface; and an electronic component embedded via a resin in a through-hole extending through the core substrate between the front surface and the back surface of the core substrate. The printed wiring substrate is characterized in that the electronic component has an electrode projecting from at least either an upper end or a lower end thereof and the resin contains an inorganic filler.
In a second embodiment, the printed wiring substrate of the present invention comprises a core substrate having a front surface and a back surface; and an electronic component embedded via a resin in a recess formed in the core substrate and extending from interior of the core substrate to the front surface or the back surface of the core substrate. The printed wiring substrate is characterized in that the electronic component has an electrode projecting from at least either an upper end or a lower end thereof and the resin contains an inorganic filler.
According to the present invention, the inorganic filler reinforces the resin and lowers the coefficient of thermal expansion of the resin. Thus, no cracks are generated in the resin used to embed an electronic component. Particularly, cracking or separation is unlikely to occur at a thin resin portion adjacent to the upper or lower end of an electronic component from which an electrode projects. Thus, an electronic component can be embedded in a through-hole or a recess formed in the core substrate such that good dielectric performance and good hermetic performance are maintained. Accordingly, the electronic component can reliably function as expected, and electrical continuity can be stably established via the electrode between the electronic component and a wiring layer formed in the printed wiring substrate.
In a third embodiment, the printed wiring substrate of the present invention comprises a core substrate having a front surface and a back surface; and an electronic component embedded in the core substrate. The printed wiring substrate is characterized in that the electronic component has an electrode projecting from at least either an upper end or a lower end thereof and the core substrate contains an inorganic filler. Since the core substrate, in which an electronic component is embedded, contains a reinforcing inorganic filler, no cracks are generated in a portion of the core substrate around the electronic component. Particularly, cracking or separation becomes unlikely to occur at a thin portion of the core substrate adjacent the upper or lower end of the electronic component from which the electrode projects. Thus, the electronic component can be embedded in the core substrate such that good dielectric performance and good hermetic performance are maintained. Accordingly, the electronic component can reliably function as expected, and electrical continuity can be stably established via the electrode between the electronic component and a wiring layer formed in the printed wiring substrate.
Examples of the above-mentioned electronic component include passive components, such as capacitors, inductors, filters, and resistors; active components, such as low noise amplifiers (LNAs), transistors, semiconductor devices, and FETs; as well as SAW filters, LC filters, antenna switch modules, couplers, and diplexers. Also included are these electronic components in the form of chips, and electronic component units each composed of a plurality of these electronic components in the form of chips. Among these electronic components, electronic components of different types may be embedded in the same through-hole or recess.
Examples of the inorganic filler include crystalline silica, fused silica, alumina, and silicon nitride. However, the present invention is not limited thereto. The inorganic filler is generally added in an amount of 35-65 vol. %, preferably from 40 to 60 vol. %, and more preferably from 40 to 50 vol. %. These amounts are for the content of the inorganic filler in both the resin and core substrate.
Through addition of the above-mentioned inorganic filler to the resin, the resin can assume a coefficient of thermal expansion of not greater than 40 ppm/xc2x0 C. (zero is not included), preferably not greater than 30 ppm/xc2x0 C. (zero is not included), more preferably not greater than 25 ppm/xc2x0 C. (zero is not included), further preferably not greater than 20 ppm/xc2x0 C. (zero is not included). Thus, stress concentration derived from the difference in coefficient of thermal expansion between the resin and an embedded electronic component can be reduced. In the above ranges of coefficient of thermal expansion, the lower limit is preferably not less than 10 ppm/xc2x0 C.
In yet a fourth embodiment, the present invention provides a printed wiring substrate wherein the particle size of the inorganic filler is not greater than one-half the height of the electrode (zero is not included).
Employing the above mentioned particle size reinforces a thin resin portion or a thin portion of the core substrate adjacent to the upper or lower end of an electronic component from which an electrode projects, thereby preventing cracking or separation of the thin portion which would otherwise result from thermal expansion or contraction. Usually, an inorganic filler is unlikely to reach a thin portion of resin or core substrate adjacent to the upper or lower end of an electronic component from which an electrode projects. However, according to the present invention, since the particle diameter of the inorganic filler is relatively small as compared with the height of the electrode, the inorganic filler reliably and uniformly reaches the thin portions. Thus, the thin portions contain a sufficient amount of inorganic filler, thereby establishing uniform distribution of the coefficient of thermal expansion and thus preventing the occurrence of cracking.
When a wiring layer is to be formed on the upper and lower sides of the core substrate by way of a build-up process, the surface of the resin is roughened with an oxidizer. In the case of the printed wiring substrate of the present invention, uniform distribution of the inorganic filler allows uniform roughening of the resin. Thus, reliable adhesion can be established between the surface of the resin used for embedding an electronic component, and a wiring layer formed on the surface of the resin. The particle size of an inorganic filler denotes the maximum particle size in a particle-size distribution of the inorganic filler.
When the maximum particle size in a particle-size distribution of an inorganic filler is in excess of one-half the height of an electrode, cracking becomes likely to occur; thus, inorganic-filler particles greater than one-half the height of an electrode are eliminated. More preferably, the particle size of an inorganic filler is not greater than one-third the height of an electrode (zero is not included). Preferably, the shape of an inorganic-filler particle is substantially spherical in order to enhance the fluidity and packing density of the resin and a material for the core substrate. However, an inorganic-filler particle may be shaped such that a cross section thereof assumes the form of an ellipse having a major axis and a minor axis. Preferably, in order to attain low viscosity and high packing density of the resin, two or more kinds of inorganic fillers of different average particle sizes and particle shapes are used in combination.
In a fifth embodiment, the present invention provides a printed wiring substrate wherein the particle size of the inorganic filler is not greater than 25 xcexcm, and the height of the electrode is not lower than 50 xcexcm.
Employing the above mentioned particle size and electrode height appropriately reinforces a thin resin portion or a thin portion of the core substrate adjacent to the upper or lower end of an electronic component from which an electrode projects, thereby reliably preventing cracking or separation. A particle size of not greater than 25 xcexcm means that the maximum particle size in a particle-size distribution is not greater than 25 xcexcm (zero is not included).
When the particle size of a filler (e.g., silica filler) is in excess of 25 xcexcm, cracking is likely to occur in the thin resin portion mentioned above; thus, particle sizes greater than 25 xcexcm are excluded. Preferably, the particle size is not greater than 20 xcexcm (zero is not included). In order to attain fluidity of the resin, the lower limit of the particle size of the filler is 0.1 xcexcm or greater, preferably not lower than 0.5 xcexcm. Herein, the particle size is measured in the following manner. A projected image of a particle obtained by means of a laser diffractometer is approximated to a circle. The diameter of the circle is measured for use as the size of the particle.
When the height of the electrode is less than 50 xcexcm, cracking as mentioned above is likely to occur; thus, electrode heights less than 50 xcexcm are excluded. Preferably, in order to prevent a short circuit between electrodes, the upper limit of the height of an electrode is 100 xcexcm or less (zero is not included). The surface roughness of the electrode of an electronic component is 0.3-20 xcexcm in terms of 10-point average roughness Rz, preferably 0.5-10 xcexcm, more preferably 0.5-5 xcexcm. Such a range of surface roughness of an electrode allows the resin to be caught by pits and projections on the surface of the electrode, thereby yielding an anchoring effect and thus enhancing adhesion. No particular limitation is imposed on the method for controlling the surface roughness. Examples of such a surface-roughening method include chemical etching, micro-etching, and blackening.
The present invention also provides a method for manufacturing a printed wiring substrate adapted to manufacture a printed wiring substrate comprising a core substrate having a front surface and a back surface and an electronic component embedded via a resin in a through-hole extending through the core substrate between the front surface and back surface of the core substrate or in a recess formed in the core substrate and extending from the interior of the core substrate to the front surface or the back surface. The method comprises the steps of: inserting into the through-hole or the recess the electronic component having an electrode projecting from at least either an upper end or a lower end thereof; embedding the electronic component in the through-hole or the recess by means of a resin containing an inorganic filler; and polishing a surface of the resin for leveling so as to expose an end surface of the electrode.
The method of the present invention reinforces a thin resin portion or a thin portion of the core substrate adjacent to the upper or lower end of an electronic component from which an electrode projects, thereby reliably providing a printed wiring substrate that is not susceptible to cracking. Also, since the thin resin portion adjacent to the upper or lower end of the electronic component is reliably filled with the inorganic filler, cracking is unlikely to occur in that portion. Thus, a printed wiring substrate having electronic components embedded in a core substrate can be reliably manufactured. Herein, the term xe2x80x9cembedxe2x80x9d means, for example, to mount in place through embedding by means of the resin mentioned above, and the term xe2x80x9clevelingxe2x80x9d means, for example, to finish the surface of the resin to a substantially flat surface.
Additionally, the manufacturing method mentioned above can be a method for manufacturing a printed wiring substrate comprising a core substrate having a front surface and a back surface and an electronic component embedded via a resin in a through-hole extending through the core substrate between the front surface and back surface of the core substrate or in a recess formed in the core substrate and extending from interior of the core substrate to the front surface or the back surface. The method comprises the steps of: inserting into the through-hole or the recess the electronic component having an electrode projecting from at least either an upper end or a lower end thereof; embedding the electronic component in the through-hole or the recess by means of a resin containing an inorganic filler whose particle size is not greater than half the height of the electrode; and polishing a surface of the resin for leveling so as to expose an end surface of the electrode. Preferably, the particle size of the inorganic filler is not greater than one-third the height of the electrode. Notably, the particle size of the inorganic filler is not greater than one-half the height of the electrode after polishing.
Additionally, the present invention provides a method for manufacturing a printed wiring substrate comprising a core substrate having a front surface and a back surface and an electronic component embedded via a resin in a through-hole extending through the core substrate between the front surface and back surface of the core substrate or in a recess formed in the core substrate and extending from the interior of the core substrate to the front surface or the back surface. The method comprises the steps of: inserting into the through-hole or the recess the electronic component having an electrode projecting not less than 50 xcexcm and less than 100 xcexcm from at least either an upper end or a lower end thereof; embedding the electronic component in the through-hole or the recess by means of a resin containing an inorganic filler whose particle size is not greater than 25 xcexcm; and polishing a surface of the resin for leveling so as to expose an end surface of the electrode. Since this method allows the inorganic filler to be reliably filled into a thin resin portion adjacent the upper or lower end of the electronic component, cracking is unlikely to occur in the portion. Thus, a printed wiring substrate having electronic components embedded in a core substrate can be reliably manufactured.